1. Field of the Invention
The invention relates to an improved fuel injection system, including a pressure booster, for use in an internal combustion engine.
2. Description of the Prior Art
For better comprehension of the description and claims, some terms will now be defined: The fuel injection system of the invention can be embodied as either stroke-controlled or pressure-controlled. Within the scope of the invention, a stroke-controlled fuel injection system is to be understood to mean that the opening and closing of the injection opening with the aid of a displaceable nozzle needle is effected on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. A pressure reduction inside the control chamber causes a stroke of the nozzle needle. Alternatively, the deflection of the nozzle needle can be effected by means of a final control element (actuator). In a pressure-controlled fuel injection system according to the invention, the nozzle needle is moved by the fuel pressure, prevailing in the nozzle chamber of an injector, counter to the action of a closing force (spring), so that the injection opening is opened for an injection of the fuel from the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called the infection pressure, while the term system pressure is understood to mean the pressure at which fuel is available or kept on hand inside the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. The term leakage, or leak fuel, is understood to mean a quantity of fuel that occurs in operation of the fuel injection system (such as a guide leakage) but is not used for injection and so is returned to the fuel tank. The pressure level of this leak fuel can be a static or fixed pressure, after which the fuel is depressurized to the pressure level of the fuel tank.
Many engine manufacturers require a shallow leading edge of the pressure at the onset of the injection. Often, a boot phase is also desired in order to lower emissions. In fuel injection systems with a pressure booster, of the kind known for instance from German Patent Disclosure DE-A1-19910970, the pressure booster can be used to shape the course of injection. Thus the desired injection course can be achieved without such additional parts as deflection pistons. To vary the pressure course, the motion of the piston of the pressure booster can be used. Varying the inlet cross section to the pressure booster chamber on the low-pressure side as a function of pressure is known from U.S. Pat. No. 5,868,317. That US patent proposes controlling the inlet cross section in multiple stages.